1. Field of the Invention
The present invention relates to a method for processing a solution including heavy metal. More specifically, the present invention relates to a method for processing a waste solution produced from a plating process such as in a plating factory.
2. Description of the Prior Art
Generally, a chemical plating solution includes a heavy metal salt group, a complexing agent, a reducing agent and a pH adjusting agent. A plating solution for use in chemically plating copper includes, e.g., copper sulfate as the heavy metal salt group, EDTA or Rochelle salt as the complexing agent, formalin as the reducing agent, and sodium hydroxide as the pH adjusting agent.
When a plating process is ended using such a chemical plating solution, the plating solution is disposed as a waste solution. Heavy metal such as copper metal and a complexing agent are dissolved in the plating waste solution in the form of a complex salt, i.e. a heavy metal complex salt which is extremely stable.
A conventional method for processing a plating waste solution including heavy metal complex salt comprises (1) a method of using ion-exchange resin, (2) a method of using a calcium salt group of such as calcium hydrate, calcium chloride or the like, or other precipitant or flocculant, and the like.
However, the former method (1) can not remove the heavy metal in the complex salt. On the other hand, the latter method (2) is a method of adding a precipitant or flocculant in a waste solution, thereby to cause cohesion of a heavy metal complex salt, whereby the heavy metal complex salt is deposited. According to the latter method, most heavy metal is recovered in a state of sediment. The heavy metal which could not be included in the sediment is discharged by diluting the same using a large quantity of water to reduce the concentration of the heavy metal. Thus, the latter method (2) involves various shortcomings that the heavy metal component can not be completely removed, the sludge amount after processing is increased due to the fact that large amounts of various kinds of chemical agents are used, and therefore a problem of secondary pollution is caused, and so on. Furthermore, the latter method (2) leaves unsolved a problem of treatment of produced sludge. In addition, in view of the above described diluting process, the total amount of heavy metal being discharged is not changed after all. In this context, the latter method (2) is not a complete method of processing a plating waste solution.
Thus, conventionally it was very difficult to process a waste solution including a heavy metal complex salt, such as a chemical plating waste solution, by the above described various kinds of chemical methods, thereby to separate heavy metal from the heavy metal complex salt and to remove the same, due to the fact that the heavy metal complex salt is extremely stable. Therefore, it might be possible to process a waste solution including a heavy metal complex salt, by an electrochemical method such as an electrolytic extraction method. However, in such a case many other problems are involved in that large electric power is required, oxidation decomposition of a complexing agent also occurs, and the like. Thus, this method is not preferred.